In recent years, a larger quantity of materials for metallic sheet laminated on both surfaces thereof with thermoplastic resin films such as polyester resin film have been used for can stocks such as drawn can and draw;n and stretch formed can or the like which are subjected to severe working. Today, cans formed by drawing and stretch forming and further ironing is developed from the economical view point. In general, when a severe forming is carried out on a metallic sheet laminated with oriented resin film, the orientation coefficient of the resin film after lamination should be reduced to a small value so as to improve the formability. Otherwise, the resin film would be peeled off or have cracks caused. Namely, a considerable part of the orientation of the resin film needs to be lost by the heat when the resin film is thermally bonded to a metallic sheet. A film for forming the outside wall of can body requires not so great corrosion resistance and impact resistance but requires higher formability than a film for forming the inside wall of can body when formed into a can. On the other hand, the film for forming the inside wall of can body needs greater corrosion resistance and impact resistance, since the film of the inside wall of can body comes into direct contact with the content after packed and sometimes would be damaged by outer impact.
In order to satisfy both the requirements for the properties of the inside and outside walls of can body, a certain extent of the orientation should be retained in the resin film to form the inside wall of can body, while preferably not so much of the orientation is retained in the resin film to form the outside wall of can body.
When a resin laminated metallic sheet having properties bearable to severe forming is produced using the conventional manufacturing method, it is necessary to control various manufacturing conditions such as heating temperature of metallic sheet, the composition of thermoplastic resin film to be laminated, degree of orientation of resin film, melting temperature of resin, laminating speed, laminating roll pressure or the like.
FIG. 3 shows a manufacturing method for resin laminated metallic sheet by a conventional art. As known by Laid-Open Japanese Patent Hei 4-201237, for example, a resin laminated metallic sheet is manufactured by a method in which thermoplastic resin film 102 is made to contact both surfaces of metallic sheet 101 heated in oven 105, pressed to metallic sheet 101 using a couple of laminating rolls (nip rolls) 103 and 104, thermally bonded to metallic sheet 101, a part of resin film 102 near metallic sheet 101 being melted by the heat of metallic sheet 101, cooled to a room temperature in quenching tank 108 and then dried. In such a manufacturing method of resin laminated metallic sheet, thickness of the melted portion of resin film 102 (thickness of melting layer), degree of orientation, adhesion strength to metallic sheet or the like can be controlled to some extent by taking account of heating temperature of metallic sheet, distance between oven 105 and laminating rolls 103, 104, travelling speed of metallic sheet, properties of resin such as composition, thickness, degree of orientation, or melting temperature, and suitably selecting manufacturing conditions for the lamination of resin film.
In the above mentioned conventional method, as shown in FIG. 4, in a case where resin film 102 is laminated on metallic sheet 101 having been heated in oven 105, when resin film 102 and metallic sheet 101 are pressed by laminating rolls 103, 104 wherein metallic sheet 101 is first heated in oven 105, heat is transferred from metallic sheet 101 of high temperature to laminating rolls 103, 104 of low temperature through resin film 102. Accordingly, melting layer 106 (adjacent unmelted layer 107) is formed at a surface of resin film 102 contacting the metallic sheet and at the same time resin film 102 and metallic sheet 101 are bonded together by the laminating roll pressure. After passing through between the laminating rolls 103, 104, they are free from the pressure, and the surface of laminate resin film is naturally cooled, while the heat of the metallic sheet is transferred to the whole of the resin film, thereby controlling the degree of orientation wholly in the resin film. When both members are cooled by the laminating rolls, an oriented layer is retained on the outermost surface of the resin film. Accordingly, if the travelling speed of metallic sheet is high, cooling by laminating rolls is insufficient. Therefore, it is necessary to lower the heating temperature of metallic sheet to control the degree of orientation of resin film. Thus, it is difficult to continuously manufacture a resin film laminated metallic sheet at high speed.
On the other hand, in a case where resin film is continuously laminated on metallic sheet at high speed, a melting layer of resin film to be laminated should be thick to some extent so as to have excellent adhesion during forming. One of the methods therefor may be to raise the heating temperature of metallic sheet. But in this case, cooling effect by laminating roll is insufficient, so that almost whole of the resin film is melted and consequently the strength of resin film is decreased. Moreover, another problem is caused that if the layer of resin film laminated on a metallic sheet, the whole of which is once melted when laminated, forms an inside wall of a can after formed, the laminated resin layer has easily cracks to cause corrosion of the can body by any outer impact when or after the content is packed in the can.
Thus, it would be possible even by the conventional method to manufacture a resin laminated metallic sheet having adhesion during forming fit for a laminate can which is formed by severe forming but its laminated resin layer is not peeled off, if only properties of the resin film to be laminated such as composition, thickness, degree of orientation, melting temperature are taken into account, the respective resin films for inside and outside wall of can are suitably selected, and manufacturing conditions such as heating temperature of metallic sheet, supplying speed of metallic sheet and resin film, pressure of laminating rolls are properly predetermined. However, it is extremely difficult to continuously manufacture such a resin film laminated metallic sheet at high speed. Moreover, when the traveling speed of metallic sheet is increased or decreased to perform the continuous manufacturing, the laminated resin film has uneven orientation, which results in the partial peeling-off of the laminated resin film when the laminate is intensely formed. Consequently, the economical efficiency and productivity of the resin laminated metallic sheet which should have superior adhesion during forming and corrosion resistance is decreased and the continuous manufacturing thereof cannot be smoothly performed.
It is an object of the present invention to solve the problems encountered in the prior art resin laminated metallic sheet, the manufacturing method and apparatus therefor, and to provide a resin laminated metallic sheet having more excellent adhesion during forming, corrosion resistance, impact resistance, and so on and also manufacturing method and manufacturing apparatus for continuously producing the same at high speed while ensuring the qualities thereof even when traveling speed of metallic sheet is varied.